The present invention refers to a control device for varying the current flow through a load, in particular to intensity-modulation control device for decorative light sets such as Christmas tree light sets.
A simple light control device is known to be the bimetal flasher in which the different expansion coefficient of two metals of a strip is used to open and close the supply of power to a light string circuit to create a twinkling effect. Such flashers, however, do not allow special effects, such as fading-in or fading-out.
For creating fading or sequential moving effects, various other control systems have been proposed like e.g. described in U.S. Pat. No. 4,125,781 which refers to a Christmas tree lighting control to continuously change light intensities between strings through cooperation of contact pairs of duty cycle units with respective resistor disks driven by a motor. The disks are of varying configuration so that depending on the contact surface of the disks between the associated contact pairs, the resistance is altered to thereby change the duty cycle rate and thus the light intensity of the light strings. The duty cycle units may be variously constructed to include a SCR or DIAC and TRIAC combination. Apart from the high costs of such devices, the use of a gear motor as described in the U.S. Pat. No. 4,125,781 or also in U.S. Pat. No. 3,944,878 is undesired because of all the moving parts which will wear out and thus reduce the life of the device.
Other control systems describe the use of beat oscillators and digital technique to attain a desired light modulation. For example, U.S. Pat. No. 3,793,531 describes an electronic tree light controller which regulates the flow of electric power to lights or other loads by employing an electronic circuitry in which a solid state switch is used. The trigger actuation of the solid switch is made responsive to the difference frequency or an oscillator frequency beating against line frequency.
In control devices which utilize a beat oscillator, the beat signal of an adjustable oscillator and the 60 Hz AC line power is used to yield a sub-Hertz order of fading rate in the output such as e.g. 0.1 Hz. As a result, the fading rate is quite sensitive even to a small fraction of the frequency drift in the power line as well as in the oscillator. For example, in the case of a beat frequency of 0.1 Hz, a 1% drift in the line or oscillator frequency causes a significant 600% variation in the fading rate. As a result, such a device should be made with an external adjustment for permitting a user to manually adjust the beat frequency as it drifts away from the desired fading rate.
It was also proposed to employ two oscillators to beat each other for generating the fading rate through a digital circuit so as to produce sequential output signals. Such a system has the same drawbacks as previously outlined and thus are not reliable and stable as an external adjustment is required for compensating the drift in the power frequency for the triac circuits. Furthermore, the use of digital techniques renders such devices rather complicated in comparison to analog circuits in view of the required extra interfaces.